In the first case, the accidental intrusion of foreign material between
the jack head and the iron caused the jack to take its bearings on the
flange above its normal position opposite the web of the ring, and
resulted usually in the breaking out of a piece of the flange or in
several radiating cracks with or without a depression of the flange.
These breaks were very characteristic, and the cause was readily
recognizable, even though the intruding substance was not actually
observed.

In the second case, the working of a hard piece of metal, such as a
small tool, into the annular space between the iron and the tail of the
shield, where it was caught on the bead and dragged along as the shield
advanced, was the known cause of a number of broken segments. Such
breaks had no particular characteristic, but were usually close above
the line of travel of the lost tool or metal. Their cause was determined
by the finding of a heavy score on the underside of the segment or the
discovery of the tool wedged in the tail of the shield or lying under
the broken plate when it was removed. It is probable that a number of
breaks ascribed to unknown causes should be placed in this class.

The third cause includes the largest number of breaks, and, while
difficult to define closely, is the most interesting. Broadly speaking,
the breaks resulted from the movements of the shield in relation to the
position of the tunnel lining. While shoving through soft ground, it was
frequently difficult to apply sufficient power to the lower jacks to
complete the full shove of 30 in. on the desired alignment. The shield,
therefore, was driven upward at the beginning of the shove, and, as the
sand packed in front of the shield and more power was required, it was
furnished by applying the upper jacks. The top of the shield was slowly
pushed over, and, at the close of the shove, the desired position had